1. Field of the Invention
This invention relates to an optical disk device which reads a signal recorded on a disk shaped optical recording medium (hereinafter referred to as an optical disk) such as a compact disk (CD), a digital versatile disk (DVD) and so on.
2. Description of the Related Art
An information reproducing device known as an optical disk device in the related art has: rotating means for rotating an optical disk; focusing means having an objective lens for converging an optical beam to be applied to the optical disk on a focusing position; servo control means including a focus servo system and a tracking servo system for controlling a position of the objective lens in the focusing means; motor control means for controlling a number of revolutions of the rotating means; memory for storing data of the rotational speed of the optical disk; and a central processing unit (hereinafter referred to as a CPU) for controlling the servo control means and the motor control means (see, for example, a patent document 1, Japanese Unexamined Patent Publication No. 63-138531, claims, FIG. 1). In this information reproducing device, a light beam spot of the optical beam emitted from an optical head of the focusing means is projected through the objective lens to a recording track of the rotating optical disk and the light beam is moved in the radial direction of the optical disk so as to trace the recording track thereby to optically reproduce information in the recording track.
At this point, the optical disk itself usually has “a warp” and hence it causes a surface swing when it is rotated. Although an allowable amount of surface swing is regulated within a predetermined range by a standard specification, in order to correctly reproduce information in the recording track of the optical disk with the allowable surface swing, it is necessary to correctly bring the optical beam from the optical head of the focusing means to a focusing position on the recording layer of the optical disk which has the surface swing. For this reason, in the information reproducing device in the related art, a focus servo control is performed by which the position of a lens with respect to the recording layer of the optical disk is controlled during reproduction in the direction of the optical axis of the optical beam. In this regard, before performing this focus servo control, every time a disk is loaded on a device and every time reproducing a disk which has already been loaded on a device is started, an operation is performed in which an objective lens in the focusing means is moved within the controllable range of the focus servo control while controlling its moving speed and a focus loop filter is actuated thereby to obtain focus, that is, the so-called focus pull-in operation.
In recent years, however, there has been a tendency to greatly increase the rotational speed of the optical disk at the time of reproducing information. In a case where the focus pull-in operation is performed on the optical disk rotating at such a high speed, when the amount of surface swing of the optical disk which is rotated at the high speed is small, the focus pull-in operation can be successfully performed, but when the amount of surface swing is large, the relative speed of the surface swing of the optical disk to the lens will is made large, so that there is a possibility that the focus pull-in operation will not be successfully completed depending on a timing.
Thus, in the information reproducing device in the related art, rotating means is controlled in such a way that when the distance between the optical disk and the focusing means is brought into the range in which focus can be obtained, the optical disk is rotated at a slower speed than the rotational speed when information is recorded into or reproduced from the optical disk. That is, when the focus pull-in operation is performed, the optical disk is rotated at a slower speed than the rotational speed during recording or reproducing the information.
Next, the focus pull-in operation will be described.
First, after a start-up job of the information reproducing device in the related art has been completed, the optical disk is rotated at a predetermined rotational speed (initial value) which is slower than a rotational speed during recording or reproducing the information, by the rotating means initially set by the motor control means. When the focus servo system of the servo control means is started in this state by a focus pull-in command from logic operation means, the focus servo system reciprocates (moves up and down) the lens along a direction of its optical axis by means of data of the moving speed of the lens and the amount of movement of the lens which are initially set, thereby to perform the focus pull-in operation. At this time, in a case when the logic operation means determines that the focus pull-in operation is not successfully performed, the focus pull-in operation is repeatedly performed as a recovery job under the same conditions until the logic operation means determines that the focus pull-in operation is successfully performed.
In this respect, in a case when the ambient temperature of the information reproducing device is low, a movement of the lens along the direction of its optical axis is made slower as compared with a case where the ambient temperature is high, so that even if the same amount of movement of the lens is set to the servo control means at the low ambient temperature and at the high ambient temperature, the actual amount of movement of the lens is made smaller at the low ambient temperature than at the high ambient temperature. For this reason, even if the focus pull-in operation is successfully completed at the high ambient temperature, the focus pull-in operation sometimes is not successfully completed at a low ambient temperature.
The information reproducing device in the related art has the above described structure and performs the recovery job under the same conditions even after a case when it is determined that the focus pull-in operation is not successfully performed. Therefore, there is presented a problem that since the rotational speed of the optical disk is initially set at a low speed in the initial focus pull-in operation, a start-up time is extended even when an amount of the surface swing is small.
Moreover, if the amount of movement of the lens is adjusted so as to be suited to the low ambient temperature in the information reproducing device in the related art, in a case when it is determined that the focus pull-in operation is not successfully performed for some reason at the high ambient temperature, there is presented problems that the lens might be brought into contact with the surface of the optical disk to cause a damage to the recording layer of the optical disk and at the same time that the start-up time is extended also in this case.